1. Field of the Invention
The present invention relates generally to soil remediation equipment and, particularly, without limitation, to such equipment utilizing a combustion chamber in a rotating drum.
2. Description of the Related Art
Remediation of contaminated soil is, and will continue to be for the foreseeable future, a large and growing industry. For example, soil containing hydrocarbons due to service station ground leakage must be cleaned or removed before the property can be transferred.
The generally applied method of remediating contaminated soil is to heat the soil with a large combustion heater to a sufficiently elevated temperature, such as approximately 400.degree.-700.degree. F., in an inclined rotating drum, that gravitationally urges the soil continuously therethrough. The elevated temperature causes the contaminants--typically hydrocarbons, both short chain and long chain--to be released from the soil by vaporization.
The vapors, containing combustion products from the burner used to elevate the soil temperature and the contaminants, are usually directed, after filtering, into an afterburner. The afterburner generally comprises a second large burner that, in conjunction with the combustion properties of the contaminants, further elevate the temperature of the vapors in order to break down and oxidize contaminants remaining in the vapors. Short-chain or light hydrocarbons, such as those arising from service station leakage, vaporize in the range of approximately 300.degree.-500 .degree. F. and require afterburning at approximately 1600.degree. F. Unfortunately, soil that has been de-contaminated generally must be discarded. Because of the high temperatures involved, substantially all nutrients in the soil are generally destroyed during remediation and, therefore, the soil is generally not conducive for growing vegetation.
For recycling asphalt materials for the production of hot mix asphalt, excessive heating of asphalt compositions may result in a substantial air pollution control problem, known as "blue-smoke", caused when hydrocarbon constituents of asphalt are driven off and released into the atmosphere. The presence of asphalt in the recycle material creates essentially the same problems in asphalt production as does the presence of liquid asphalt. The volatile components of the asphalt are released upon exposure to high temperatures and may be carried in the exhaust gases to the air pollution control equipment, typically a baghouse. Within the baghouse, the blue-smoke or tiny particles of asphalt will condense on the filter bags reducing their efficiency and presenting a serious fire hazard. The useful life of the fabric filter used in the baghouse is also reduced when contaminated with asphalt.
Various systems have been developed for using recycle asphalt materials in the production of hot mix asphalt. For example, U.S. Pat. No. 5,470,146 provides a hot mix asphalt plant wherein a drum mixer isolates liquid asphalt and recycle asphalt materials, if used, from the radiant heat flux of a burner flame and from a hot gas stream produced by the burner to thereby reduce air pollution emission, including reducing the amount of hydrocarbons entrained in the hot gas stream and carried to the air pollution control equipment. Fortunately, some of the volatile contaminants of contaminated soil are more readily vaporizable and oxidizable than the asphalt constituents of liquid asphalt and recycle asphalt materials. As a result and in regard to air pollution control equipment, the precautions for remediating soil contaminated with highly volatile contaminants, such as gasoline, are not as burdensome as those for preventing blue-smoke production from asphaltic constituents.
One attempt to dispose of remediated soil is disclosed in U.S. Pat. No. 5,378,059 wherein remediated soil is mixed with other constituents for producing hot mix asphalt for asphalt paving and the like. The '059 apparatus includes a rotating drum having a lower end thereof mounted in a fixed sleeve such that an annular chamber is formed between the rotating drum and the fixed sleeve. Contaminated soil, recycle asphalt material and various ingredients are introduced into the annular chamber formed by the fixed sleeve to be combined with heated aggregate also introduced into the annular chamber from an interior chamber of the rotating drum. Performance of the '059 apparatus is dependent upon the ability of a plurality of mixing blades, mounted on the outer surface of the rotating drum and spaced within the annular chamber formed by the fixed sleeve, to reliably mix the heated aggregate with the contaminated soil, recycle asphalt and other ingredients while simultaneously urging all of those materials up the incline of the fixed sleeve. All materials being conveyed through the annular chamber formed by the fixed sleeve are heated by contact with the heated aggregate, by conductive heat transfer from the mixing blades, and by radiant heat transfer from the outer peripheral surface of the rotating drum.
What is needed is a system for concurrently remediating contaminated soil and producing hot mix asphalt wherein virgin aggregate is dried and heated, contaminated soil is remediated, and hot mix asphalt is produced from the remediated soil, recycle asphalt products, and customary constituents in an isolated mixing zone wherein available thermal energy for such hot mix asphalt production is provided substantially only by thermal energy contained within the materials introduced into the mixing zone--all within a single rotating drum.